This invention relates to apparatus employable for grinding materials and, more specifically, to an imp mill particularly suited for the fine and medium-fine grinding of non-metallic minerals and coal, which is provided with adjustment means whereby the degree of fineness to which the material is ground can be adjusted.
It has long been known in the prior art to provide apparatus employable for purposes of effecting the grinding of materials. More specifically, the prior art is replete with examples of various types of apparatus that have been used heretofore to effect the grinding of a multiplicity of different kinds of materials. In this regard, in many instances discernible differences of a structural nature can be found to exist between individual ones of the aforesaid apparatus. The existence of such differences is, in turn, attributable for the most part, to the diverse functional requirements that are associated with the specific applications in which such apparatus are designed to be employed. For instance, in the selection of the particular type of apparatus that is to be utilized for a specific application one of the principal factors to which consideration must be given is that of the nature of the material that is to be ground in the apparatus. Another factor to which consideration must be given is that of the fineness to which it is desired to grind the material.
One form of apparatus, which has long been utilized in the prior art, as a means of effecting the grinding of a variety of different types of material is that commonly referred by applicant's assignee as an imp mill. Namely, the latter, which, generally speaking, may be viewed as being a particular type of hammermill, is one of the forms of pulverizers commonly employed for reducing the size of minerals, organics and chemicals. In accord with the mode of operation thereof, it is normally swept by air to remove the pulverized, i.e., ground, product, which is ordinarily reduced therein in size from three-quarter inch lumps to finenesses wherein all of the pulverized product is capable of passing through a 20 mesh screen to finenesses wherein all of the pulverized product is capable of passing through a 325 mesh screen. Basically, such mills consist of swing-hammers that are suitably pinned to discs, which, in turn, are attached to a rotor shaft that is housed in a cylindrical grinding chamber. Commonly, the rotor shaft is supported in a horizontal plane, and the grinding chamber has an air inlet provided on one end and an air outlet provided on the other end so that the air, which sweeps through the mill, flows parallel to the rotor shaft.
One of the earliest uses to which imp mills were put was that of the pulverization of coal, and particularly in those applications wherein it was desired to pulverize the coal for direct firing. However, as these systems in which direct firing of pulverized coal is employed have grown larger, there has been a concomitant increase in the amount of pulverized coal that is required to be supplied thereto. Thus, it has now been found that in many instances the demands of such direct firing systems for pulverized coal exceeds the capacity of the imp mills to provide the pulverized coal. Consequently, other forms of pulverizing equipment are now being utilized in such applications, i.e., those which require larger amounts of pulverized coal. Nonetheless, imp mills continue to be employed to provide the amounts of pulverized coal required in smaller capacity installations, such as those wherein the direct firing of the pulverized coal takes place in rotary kilns, rotary dryers or industrial furnaces.
Another application in which it has been known to utilize imp mills is in the clay fields of Kentucky and Tennessee wherein the mills are employed to effect the grinding of the clay materials, which are to be found thereat. Imp mills are also widely used in the complete processing of such products as organic insecticides, soya flour, starches, litharge for storage batteries, phosphate materials, synthetic resins, potassium compounds and in literally dozens of other applications in which precision grinding and drying are an important part of the production process.
There are many factors that affect the fineness of grind that an imp mill is capable of providing. Among these factors there are to be found included the following: mill speed, size of the swing-hammers, the number of swing-hammers, and the clearance which exists between the tips of the swing-hammers and the grinding chamber. Any or all of the above may be varied separately or collectively to effect a change in the grinding characteristics of the mill. However, the most critical factor, which determines the mill performance in terms of its grinding characteristics, in the relationship that the grinding chamber periphery bears to the air outlet.
Namely, if the entrance to the air outlet is in line, i.e., coextensive, with the periphery of the grinding chamber, the mill is incapable of producing a fine grind of material irrespective of whether the other above-recited factors are varied. On the other hand, if the entrance to the air outlet is offset towards the center line of the mill, i.e., is not coextensive with the periphery of the grinding chamber, the mill will produce a very fine grind and will be incapable of providing a coarse grind. Essentially, the reason why the imp mills performs in the aforedescribed manner is that in order for the material to be ground fine, it must be retained in the grinding chamber for a longer period of time. Consequently, in the situation wherein the entrance to the air outlet is coextensive with the periphery of the grinding chamber, the material once it has reached the exit side of the grinding chamber is free to flow into the air outlet, and thereby leave the grinding chamber. However, in the case wherein the entrance to the air outlet is offset towards the center line of the grinding chamber, the portion of the side wall of the grinding chamber that is formed by virtue of the aforesaid offset relationship of the air outlet to the periphery of the grinding chamber is operative as a dam, i.e., barrier or abutment, to the flow of material from the grinding chamber. Namely, some of the material upon reaching the exit side of the grinding chamber encounters for aforementioned dam and is prevented thereby from flowing into the air outlet, i.e., from exiting from the grinding chamber. Instead, the material that is blocked by the aforesaid barrier is made to remain in the grinding chamber for a longer period of time, during which it is subjected to additional grinding, before this material passes into the air outlet. Therefore, in summary, the offset in the grinding chamber-air outlet relationship becomes a retaining dam, which forces material to remain longer within the grinding chamber with the result that a finer grind of material is generated.
Heretofore, the practice commonly followed in the industry was to provide a different model of imp mill according to the fineness of grind that was desired. To this end, there existed a so-called fine grind imp mill and a so-called coarse grind imp mill. Thus, depending on the nature of the application in which the imp mill was intended to be employed, the appropriate model of imp mill would be selected for use therein. Thereafter, the specific degree of fineness that was desired, within relatively restricted limits, could be obtained by varying one or more of the plurality of factors that have been referred to previously hereinabove. However, as alluded to above earlier, because of the nature of the construction thereof, it is not possible through simply varying the afore-referenced factors, to transform an imp mill designed as a fine grind mill into a coarse grind mill or visa versa to transform a coarse grind mill into a fine grind mill.
A further disadvantage associated with the employment of such separate fine grind mills and coarse grind mills stems from the limitations inherent therein as to the extent to which variations in degree of fineness can be effected therewith. Namely, it is desirable to be able to maintain a constant degree of fineness of grind even as the mill wears. To this end, it is readily known that the fineness of the product of an imp mill will fall off rapidly with wear of the swing-hammers and grinding chamber liners. As a consequence of this, in many applications such wear may require the mill to be shut down frequently, sometimes only after a relatively few days of operation, in order to replace the swing-hammers.
Thus, there has been evidenced in the prior art a need for a new and improved imp mill that would be capable of selectively functioning as either a fine grind mill or a coarse grind mill as needed to meet the requirements of a variety of applications. Namely, rather than employing different models of imp mills depending on whether a given application involves grinding material to a fine grind or to a coarse grind, a need has existed for a single imp mill that could be utilized as either a fine grinder or a coarse grinder. Also, a need has been demonstrated for such an imp mill that would embody the additional capability of being able to be adjusted during operation as required so as to maintain a constant fineness of grind of the product as the mill wears.
It is, therefore, an object of the present invention to provide a new and improved apparatus, which is employable for grinding material.
It is another object of the present invention to provide such a new and improved apparatus, which comprises a hammermill wherein the grinding of material is effected through the use of a multiplicity of hammers.
It is still another object of the present invention to provide such a new and improved apparatus, which comprises a particular form of hammermill; namely, an imp mill that is characterized by the fact that it is particularly suited for the fine and medium-fine grinding of non-metallic minerals and coal.
A further object of the present invention is to provide such an imp mill that embodies adjustment means operative to enable the degree of fineness to which the material is to be ground to be adjusted, while the mill is in operation, i.e., obviate the necessity of having to shut down the unit to make changes therein as required in the case of prior art mills.
A still further object of the present invention is to provide such an imp mill, which is capable of selectively functioning as either a fine grind mill or a coarse grind mill.
Yet another object of the present invention is to provide such an imp mill, which embodies the capability of enabling adjustments to be made in the degree of fineness of the grind of the material such that it is possible to maintain therewith a constant fineness of grind as the mill wears.
Yet still another object of the present invention is to provide such an imp mill that is relatively inexpensive to manufacture, relatively easy to employ, and which is capable of providing reliable operation.